How can I combine work that I've done in the "video editing" and "2D Animation" tabs in the same project on a render?
In order to add a scene into the VSE with a scene strip, you'll have to make a new scene for the VSE contents, as shown in this gif(from this post: https://blender.stackexchange.com/a/184931/37272):

To render from the VSE you'll need to check this box: